1. Field of the Invention
The present invention relates to a driveline for a four-wheel drive vehicle that can switch between a two-wheel drive state and a four-wheel drive state.
2. Description of the Related Art
Some four-wheel drive vehicles can change its drive state between a two-wheel drive that is suitable for ordinary running on a high stability dry road and has good gas mileage and a four-wheel drive that is suitable for running in rain or snow or on a rough road. A driveline of such the four-wheel drive vehicle includes a 2WD/4WD switching mechanism that is provided in a transfer (power distribution mechanism) and can switch between the two-wheel drive state and the four-wheel drive state.
In the related art, for example, Japanese Patent Application Publication No. 2004-324676 discloses a 2WD/4WD switching mechanism that is operated by forming a cam groove on an outer peripheral surface of a cylindrical cam, fitting a fork to the cam groove, and rotating the cam with an actuator such as an electrically driven motor to displace the fork in an axial direction by following the shape of the cam groove.
Furthermore, the related art that includes a disconnect mechanism (ADD mechanism) that switches transmission/non-transmission of torque between one of right and left wheels (for example, front wheels) as driven wheels in the two-wheel drive state and a differential mechanism (for example, front differential) that is provided between the right and the left wheels is known. The driveline that includes such the disconnect mechanism can achieve improvement in fuel efficiency during the two-wheel drive state by not transmitting torque between the differential mechanism and one of wheels and thereby achieving a non-rotating state of a differential case or a drive axle in the differential mechanism.
Here, as the 2WD/4WD switching mechanism, when a wet clutch is employed instead of a so-called sleeve lock type, energy regenerative efficiency in deceleration of a hybrid vehicle, for example, can be improved; however, a following problem is concerned. That is, there is a problem that it is hard to achieve the improvement in switching responsivity from the two-wheel drive state to the four-wheel drive state and the improvement in fuel efficiency at the same time.
Specifically, in view of the improvement in changing responsivity, it is desirable to reduce a gap between friction plates of the wet clutch. However, when the gap between the friction plates is reduced, a shearing resistance (drag resistance) of oil or a frictional resistance due to contact of the friction plates increases, and therefore the problem arises such that losses increase. Furthermore, in the driveline that includes such the disconnect mechanism, during the two-wheel drive state, the wet clutch becomes a fully differential state, that is, rotating difference between the friction plates becomes maximum, and therefore seizure might occur if the gap between the friction plates would be small.
On the other hand, in view of the improvement in fuel efficiency or seizure prevention, it is desirable to increase the gap between the friction plates of the wet clutch. However, when the gap between the friction plates is increased, changing time that is required for changing from the two-wheel drive state to the four-wheel drive state gets longer, and therefore the problem arises such that the responsivity deteriorates.